NullReferenceException - Having the classic "Object reference not set to an instance of an object." when marking a spelling test

Question

Recently, I've been doing this coursework for my college titled "SpellingBee project". This is where a student will take a test loaded from an Access 2010 database and output it on the form.

I have an algorithm which checks the accuracy of the spelling inputted by a student, and it will give either 2 points, 1 point or 0 points depending on the conditions. <-- This can be found in 'testword' private function.

The algorithm will be used in a procedure called 'btnMarkIt', and essentially it just takes all the answers and calculate the total score out of 20.

Here's the code:

Imports System.Data
Imports System.Data.OleDb

Public Class frmTakeTest

Dim TestConnection As New OleDbConnection
Dim DtasetTest As New DataSet
Dim DtaadpTest As New OleDbDataAdapter
Dim SqlCmdBldTest As New OleDbCommandBuilder(DtaadpTest)

Dim CurrentRowNo As Integer = -1
Dim MarkTest As Boolean = False
Dim TakeTestNow As Boolean = False
Dim ViewOnly As Boolean
Dim totalMarks As Integer

Private Sub frmTakeTest_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

    Dim DriveLetter As Char = Application.StartupPath.Substring(0, 1)

    TestConnection.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" & DriveLetter & ":\Coursework - Computing\prj_computer-course\Jamie - Coursework\CourseworkDB.accdb"
    DtaadpTest.SelectCommand = New OleDbCommand
    DtaadpTest.SelectCommand.Connection = TestConnection
    DtaadpTest.SelectCommand.CommandText = "SELECT * FROM tbl_test"
    DtaadpTest.Fill(DtasetTest, "tblTakeTest")

    ViewOnly = True
    Protect()

End Sub

Private Sub DigitsOnly(ByRef Character As Char)

    'Validate character input: digit keys only
    If Char.IsDigit(Character) = False And Char.IsControl(Character) = False Then
        MessageBox.Show("Digits only.", "Validation Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
        'Stop invalid character appearing in field
        Character = Nothing
    End If

End Sub

Private Sub DisplayAccount()

    'Purpose: Display a test when the user adds or edit a new record.
    If DtasetTest.Tables("tblTakeTest").Rows.Count > 0 Then

        txtDef1.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def1").ToString
        txtDef2.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def2").ToString
        txtDef3.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def3").ToString
        txtDef4.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def4").ToString
        txtDef5.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def5").ToString
        txtDef6.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def6").ToString
        txtDef7.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def7").ToString
        txtDef8.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def8").ToString
        txtDef9.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def9").ToString
        txtDef10.Text = DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Def10").ToString

    End If

End Sub

Private Sub Protect()

    'Purpose: To enable/disable screen objects depending on whether ViewOnly is true or false
    If TakeTestNow = True Then
        txtDef1.ReadOnly = True
        txtDef2.ReadOnly = True
        txtDef3.ReadOnly = True
        txtDef4.ReadOnly = True
        txtDef5.ReadOnly = True
        txtDef6.ReadOnly = True
        txtDef7.ReadOnly = True
        txtDef8.ReadOnly = True
        txtDef9.ReadOnly = True
        txtDef10.ReadOnly = True
    Else
        txtDef1.ReadOnly = ViewOnly
        txtDef2.ReadOnly = ViewOnly
        txtDef3.ReadOnly = ViewOnly
        txtDef4.ReadOnly = ViewOnly
        txtDef5.ReadOnly = ViewOnly
        txtDef6.ReadOnly = ViewOnly
        txtDef7.ReadOnly = ViewOnly
        txtDef8.ReadOnly = ViewOnly
        txtDef9.ReadOnly = ViewOnly
        txtDef10.ReadOnly = ViewOnly
    End If

    txtAns1.ReadOnly = ViewOnly
    txtAns2.ReadOnly = ViewOnly
    txtAns3.ReadOnly = ViewOnly
    txtAns4.ReadOnly = ViewOnly
    txtAns5.ReadOnly = ViewOnly
    txtAns6.ReadOnly = ViewOnly
    txtAns7.ReadOnly = ViewOnly
    txtAns8.ReadOnly = ViewOnly
    txtAns9.ReadOnly = ViewOnly
    txtAns10.ReadOnly = ViewOnly

    txtSearch.ReadOnly = Not ViewOnly

    btnLoadTest.Enabled = ViewOnly
    btnMarkIt.Enabled = Not ViewOnly
    btnSubmit.Enabled = Not ViewOnly
    btnPrintPreview.Enabled = Not ViewOnly
    btnPrint.Enabled = Not ViewOnly

End Sub

Private Sub CloseToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CloseToolStripMenuItem.Click

    Dim Result = MessageBox.Show("Are you sure you want to quite? Any information entered will not be saved.", "Warning", MessageBoxButtons.YesNo, MessageBoxIcon.Warning)

    If Result = Windows.Forms.DialogResult.Yes Then
        Me.Close()
    Else
        'Do Nothing
    End If

End Sub

Private Sub btnLoadTest_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnLoadTest.Click

    'Purpose: Load an existing test from the database and output it to the user
    If txtSearch.Text = Nothing Then
        MessageBox.Show("Please enter an ID number.", "Search Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
    Else
        If IsNumeric(txtSearch.Text) = True Then
            DtaadpTest.SelectCommand.CommandText = "SELECT * FROM tbl_test WHERE TestID = " & txtSearch.Text
        End If
        DtasetTest.Tables("tblTakeTest").Clear()
        DtaadpTest.Fill(DtasetTest, "tblTakeTest")
        If DtasetTest.Tables("tblTakeTest").Rows.Count = 0 Then
            MessageBox.Show("Test not found.", "Search Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
            ViewOnly = True
        Else
            Dim Result = MessageBox.Show("Test found! Would you like to take it now?", "Question", MessageBoxButtons.YesNo, MessageBoxIcon.Question)

            If Result = Windows.Forms.DialogResult.Yes Then
                CurrentRowNo = 0
                TakeTestNow = True
                DisplayAccount()
                ViewOnly = False
                Protect()
            Else
                'Do Nothing
            End If
        End If
    End If

End Sub

Private Sub txtSearch_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtSearch.KeyPress

    DigitsOnly(e.KeyChar)

End Sub

Private Sub btnMarkIt_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnMarkIt.Click

    Dim marks(9) As Integer

    For i = 0 To 9
        marks(i) = testWord(Controls("txtAns" & i + 1).Text, DtasetTest.Tables("tblTakeTest").Rows(0).Item("Ans" & i + 1))
    Next i

    Dim msgText As String = ""
    For i = 0 To 9
        msgText += DtasetTest.Tables("tblTakeTest").Rows(CurrentRowNo).Item("Ans" & i + 1) & " " & marks(i) & " points" & vbNewLine
    Next
    MsgBox(msgText)

    totalMarks = CInt(marks(0) + marks(1) + marks(2) + marks(3) + marks(4) + marks(5) + marks(6) + marks(7) + marks(8) + marks(9))

    txtTotalMark.Text = totalMarks

End Sub

Private Function testWord(ByVal inputWord As String, ByVal actualWord As String)

    Dim lengthScore, accuracyScore, accuracyTally As Integer
    inputWord = inputWord.ToLower
    actualWord = actualWord.ToLower

    'Length
    If inputWord.Length = actualWord.Length Then
        lengthScore = 2
    ElseIf inputWord.Length = actualWord.Length + 1 Or inputWord.Length = actualWord.Length - 1 Then
        lengthScore = 1
    Else
        lengthScore = 0
    End If

    'Accuracy
    Dim inputArray() As Char = inputWord.ToCharArray
    Dim actualArray() As Char = actualWord.ToCharArray
    Dim found As Boolean
    For i = 0 To inputArray.Length - 1
        found = False
        If actualArray.Length > i Then
            If inputArray(i) = actualArray(i) Then
                accuracyTally = accuracyTally + 2
                found = True
            End If
        End If
        If found = False And i > 0 And i <= (actualArray.Length) Then
            If inputArray(i) = actualArray(i - 1) Then
                accuracyTally = accuracyTally + 1
                found = True
            End If
        End If
        If found = False And i < (actualArray.Length - 1) Then
            If inputArray(i) = actualArray(i + 1) Then
                accuracyTally = accuracyTally + 1
                found = True
            End If
        End If
    Next i

    'Add up
    Dim accMax As Integer = inputWord.Length * 2
    Dim accPerc As Integer
    If accuracyTally > 0 Then
        accPerc = CInt((accuracyTally / accMax) * 100)
    Else
        accPerc = 0
    End If
    If accPerc = 100 Then
        accuracyScore = 2
    ElseIf accPerc > 70 Then
        accuracyScore = 1
    Else
        accuracyScore = 0
    End If

    If lengthScore = 2 And accuracyScore = 2 Then
        Return 2
    ElseIf lengthScore > 0 And accuracyScore > 0 Then
        Return 1
    Else
        Return 0
    End If

End Function
End Class

*Note: The error occurs in the 'btnMarkIt' procedure:

For i = 0 To 9
    marks(i) = testWord(Controls("txtAns" & i + 1).Text, DtasetTest.Tables("tblTakeTest").Rows(0).Item("Ans" & i + 1))
Next i

Answer 1

Please try:

For i as Integer = 0 To 9
    marks(i) = testWord(Controls("txtAns" & i + 1).Text, DtasetTest.Tables("tblTakeTest").Rows(0).Item("Ans" & i + 1))
Next i